NetNewsWire/Multiplatform/iOS/Article/ImageScrollView.swift

//
//  ImageScrollView.swift
//  Multiplatform iOS
//
//  Created by Maurice Parker on 7/6/20.
//  Copyright © 2020 Ranchero Software. All rights reserved.
//

import UIKit

@objc public protocol ImageScrollViewDelegate: UIScrollViewDelegate {
	func imageScrollViewDidGestureSwipeUp(imageScrollView: ImageScrollView)
	func imageScrollViewDidGestureSwipeDown(imageScrollView: ImageScrollView)
}

open class ImageScrollView: UIScrollView {
	
	@objc public enum ScaleMode: Int {
		case aspectFill
		case aspectFit
		case widthFill
		case heightFill
	}
	
	@objc public enum Offset: Int {
		case begining
		case center
	}
	
	static let kZoomInFactorFromMinWhenDoubleTap: CGFloat = 2
	
	@objc open var imageContentMode: ScaleMode = .widthFill
	@objc open var initialOffset: Offset = .begining
	
	@objc public private(set) var zoomView: UIImageView? = nil
	
	@objc open weak var imageScrollViewDelegate: ImageScrollViewDelegate?
	
	var imageSize: CGSize = CGSize.zero
	private var pointToCenterAfterResize: CGPoint = CGPoint.zero
	private var scaleToRestoreAfterResize: CGFloat = 1.0
	var maxScaleFromMinScale: CGFloat = 3.0
	
	var zoomedFrame: CGRect {
		return zoomView?.frame ?? CGRect.zero
	}
	
	override open var frame: CGRect {
		willSet {
			if frame.equalTo(newValue) == false && newValue.equalTo(CGRect.zero) == false && imageSize.equalTo(CGSize.zero) == false {
				prepareToResize()
			}
		}
		
		didSet {
			if frame.equalTo(oldValue) == false && frame.equalTo(CGRect.zero) == false && imageSize.equalTo(CGSize.zero) == false {
				recoverFromResizing()
			}
		}
	}
	
	override public init(frame: CGRect) {
		super.init(frame: frame)
		
		initialize()
	}
	
	required public init?(coder aDecoder: NSCoder) {
		super.init(coder: aDecoder)
		
		initialize()
	}
	
	private func initialize() {
		showsVerticalScrollIndicator = false
		showsHorizontalScrollIndicator = false
		bouncesZoom = true
		decelerationRate = UIScrollView.DecelerationRate.fast
		delegate = self
	}
	
	@objc public func adjustFrameToCenter() {
		
		guard let unwrappedZoomView = zoomView else {
			return
		}
		
		var frameToCenter = unwrappedZoomView.frame
		
		// center horizontally
		if frameToCenter.size.width < bounds.width {
			frameToCenter.origin.x = (bounds.width - frameToCenter.size.width) / 2
		} else {
			frameToCenter.origin.x = 0
		}
		
		// center vertically
		if frameToCenter.size.height < bounds.height {
			frameToCenter.origin.y = (bounds.height - frameToCenter.size.height) / 2
		} else {
			frameToCenter.origin.y = 0
		}
		
		unwrappedZoomView.frame = frameToCenter
	}
	
	private func prepareToResize() {
		let boundsCenter = CGPoint(x: bounds.midX, y: bounds.midY)
		pointToCenterAfterResize = convert(boundsCenter, to: zoomView)
		
		scaleToRestoreAfterResize = zoomScale
		
		// If we're at the minimum zoom scale, preserve that by returning 0, which will be converted to the minimum
		// allowable scale when the scale is restored.
		if scaleToRestoreAfterResize <= minimumZoomScale + CGFloat(Float.ulpOfOne) {
			scaleToRestoreAfterResize = 0
		}
	}
	
	private func recoverFromResizing() {
		setMaxMinZoomScalesForCurrentBounds()
		
		// restore zoom scale, first making sure it is within the allowable range.
		let maxZoomScale = max(minimumZoomScale, scaleToRestoreAfterResize)
		zoomScale = min(maximumZoomScale, maxZoomScale)
		
		// restore center point, first making sure it is within the allowable range.
		
		// convert our desired center point back to our own coordinate space
		let boundsCenter = convert(pointToCenterAfterResize, to: zoomView)
		
		// calculate the content offset that would yield that center point
		var offset = CGPoint(x: boundsCenter.x - bounds.size.width/2.0, y: boundsCenter.y - bounds.size.height/2.0)
		
		// restore offset, adjusted to be within the allowable range
		let maxOffset = maximumContentOffset()
		let minOffset = minimumContentOffset()
		
		var realMaxOffset = min(maxOffset.x, offset.x)
		offset.x = max(minOffset.x, realMaxOffset)
		
		realMaxOffset = min(maxOffset.y, offset.y)
		offset.y = max(minOffset.y, realMaxOffset)
		
		contentOffset = offset
	}
	
	private func maximumContentOffset() -> CGPoint {
		return CGPoint(x: contentSize.width - bounds.width,y:contentSize.height - bounds.height)
	}
	
	private func minimumContentOffset() -> CGPoint {
		return CGPoint.zero
	}
	
	// MARK: - Set up
	
	open func setup() {
		var topSupperView = superview
		
		while topSupperView?.superview != nil {
			topSupperView = topSupperView?.superview
		}
		
		// Make sure views have already layout with precise frame
		topSupperView?.layoutIfNeeded()
	}
	
	// MARK: - Display image
	
	@objc open func display(image: UIImage) {
		
		if let zoomView = zoomView {
			zoomView.removeFromSuperview()
		}
		
		zoomView = UIImageView(image: image)
		zoomView!.isUserInteractionEnabled = true
		addSubview(zoomView!)
		
		let tapGesture = UITapGestureRecognizer(target: self, action: #selector(doubleTapGestureRecognizer(_:)))
		tapGesture.numberOfTapsRequired = 2
		zoomView!.addGestureRecognizer(tapGesture)
		
		let downSwipeGesture = UISwipeGestureRecognizer(target: self, action: #selector(swipeUpGestureRecognizer(_:)))
		downSwipeGesture.direction = .down
		zoomView!.addGestureRecognizer(downSwipeGesture)
		
		let upSwipeGesture = UISwipeGestureRecognizer(target: self, action: #selector(swipeDownGestureRecognizer(_:)))
		upSwipeGesture.direction = .up
		zoomView!.addGestureRecognizer(upSwipeGesture)
		
		configureImageForSize(image.size)
		adjustFrameToCenter()
	}
	
	private func configureImageForSize(_ size: CGSize) {
		imageSize = size
		contentSize = imageSize
		setMaxMinZoomScalesForCurrentBounds()
		zoomScale = minimumZoomScale
		
		switch initialOffset {
		case .begining:
			contentOffset =  CGPoint.zero
		case .center:
			let xOffset = contentSize.width < bounds.width ? 0 : (contentSize.width - bounds.width)/2
			let yOffset = contentSize.height < bounds.height ? 0 : (contentSize.height - bounds.height)/2
			
			switch imageContentMode {
			case .aspectFit:
				contentOffset =  CGPoint.zero
			case .aspectFill:
				contentOffset = CGPoint(x: xOffset, y: yOffset)
			case .heightFill:
				contentOffset = CGPoint(x: xOffset, y: 0)
			case .widthFill:
				contentOffset = CGPoint(x: 0, y: yOffset)
			}
		}
	}
	
	private func setMaxMinZoomScalesForCurrentBounds() {
		// calculate min/max zoomscale
		let xScale = bounds.width / imageSize.width    // the scale needed to perfectly fit the image width-wise
		let yScale = bounds.height / imageSize.height   // the scale needed to perfectly fit the image height-wise
		
		var minScale: CGFloat = 1
		
		switch imageContentMode {
		case .aspectFill:
			minScale = max(xScale, yScale)
		case .aspectFit:
			minScale = min(xScale, yScale)
		case .widthFill:
			minScale = xScale
		case .heightFill:
			minScale = yScale
		}
		
		
		let maxScale = maxScaleFromMinScale*minScale
		
		// don't let minScale exceed maxScale. (If the image is smaller than the screen, we don't want to force it to be zoomed.)
		if minScale > maxScale {
			minScale = maxScale
		}
		
		maximumZoomScale = maxScale
		minimumZoomScale = minScale // * 0.999 // the multiply factor to prevent user cannot scroll page while they use this control in UIPageViewController
	}
	
	// MARK: - Gesture
	
	@objc func doubleTapGestureRecognizer(_ gestureRecognizer: UIGestureRecognizer) {
		// zoom out if it bigger than middle scale point. Else, zoom in
		if zoomScale >= maximumZoomScale / 2.0 {
			setZoomScale(minimumZoomScale, animated: true)
		} else {
			let center = gestureRecognizer.location(in: gestureRecognizer.view)
			let zoomRect = zoomRectForScale(ImageScrollView.kZoomInFactorFromMinWhenDoubleTap * minimumZoomScale, center: center)
			zoom(to: zoomRect, animated: true)
		}
	}
	
	@objc func swipeUpGestureRecognizer(_ gestureRecognizer: UIGestureRecognizer) {
		if gestureRecognizer.state == .ended {
			imageScrollViewDelegate?.imageScrollViewDidGestureSwipeUp(imageScrollView: self)
		}
	}
	
	@objc func swipeDownGestureRecognizer(_ gestureRecognizer: UIGestureRecognizer) {
		if gestureRecognizer.state == .ended {
			imageScrollViewDelegate?.imageScrollViewDidGestureSwipeDown(imageScrollView: self)
		}
	}
	
	private func zoomRectForScale(_ scale: CGFloat, center: CGPoint) -> CGRect {
		var zoomRect = CGRect.zero
		
		// the zoom rect is in the content view's coordinates.
		// at a zoom scale of 1.0, it would be the size of the imageScrollView's bounds.
		// as the zoom scale decreases, so more content is visible, the size of the rect grows.
		zoomRect.size.height = frame.size.height / scale
		zoomRect.size.width  = frame.size.width  / scale
		
		// choose an origin so as to get the right center.
		zoomRect.origin.x    = center.x - (zoomRect.size.width  / 2.0)
		zoomRect.origin.y    = center.y - (zoomRect.size.height / 2.0)
		
		return zoomRect
	}
	
	open func refresh() {
		if let image = zoomView?.image {
			display(image: image)
		}
	}
	
	open func resize() {
		self.configureImageForSize(self.imageSize)
	}
}

extension ImageScrollView: UIScrollViewDelegate {
	
	public func scrollViewDidScroll(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewDidScroll?(scrollView)
	}
	
	public func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewWillBeginDragging?(scrollView)
	}
	
	public func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
		imageScrollViewDelegate?.scrollViewWillEndDragging?(scrollView, withVelocity: velocity, targetContentOffset: targetContentOffset)
	}
	
	public func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
		imageScrollViewDelegate?.scrollViewDidEndDragging?(scrollView, willDecelerate: decelerate)
	}
	
	public func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewWillBeginDecelerating?(scrollView)
	}
	
	public func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewDidEndDecelerating?(scrollView)
	}
	
	public func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewDidEndScrollingAnimation?(scrollView)
	}
	
	public func scrollViewWillBeginZooming(_ scrollView: UIScrollView, with view: UIView?) {
		imageScrollViewDelegate?.scrollViewWillBeginZooming?(scrollView, with: view)
	}
	
	public func scrollViewDidEndZooming(_ scrollView: UIScrollView, with view: UIView?, atScale scale: CGFloat) {
		imageScrollViewDelegate?.scrollViewDidEndZooming?(scrollView, with: view, atScale: scale)
	}
	
	public func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
		return false
	}
	
	@available(iOS 11.0, *)
	public func scrollViewDidChangeAdjustedContentInset(_ scrollView: UIScrollView) {
		imageScrollViewDelegate?.scrollViewDidChangeAdjustedContentInset?(scrollView)
	}
	
	public func viewForZooming(in scrollView: UIScrollView) -> UIView? {
		return zoomView
	}
	
	public func scrollViewDidZoom(_ scrollView: UIScrollView) {
		adjustFrameToCenter()
		imageScrollViewDelegate?.scrollViewDidZoom?(scrollView)
	}
	
}